Placing object on uneven surface
If you don't want to go to the expense of simulation, a rig can get you quite close:


Prepare a copy of the plate's top surface, its rim (E,right-click, S) extended, as a Shrinkwrap target.





Make a triangular face from some vertices of your toast,ShiftD duplicated, and P separated to a new object. This will be the root object, and control the gross orientation and location of the toast.





Additionally, create a flat Lattice, (1 segment in Z) which will bend the toast.





With the toast and triangle selected, triangle active and in Edit Mode with all verts selected, CtrlP parent the toast to the triangle's vertices
in Object Mode, CtrlP parent the Lattice to the triangle.
Assign Shrinkwrap modifiers projected in Z, to both the triangle and the lattice, aimed at your newly created Srinkwrap target.
Assign a Lattice modifier to the toast, targeted on  the Lattice.


Now when you move the triangle, the toast will more-or-less conform to the plate. You can tune the resolution of the Lattice and influence of the Lattice modifier to make the toast more or less soggy, and the offset in the Shrinkwrap modifiers to control the proximity of the toast to the plate.



EDIT: A duff version of the blend replaced: 1400 GMT 19 March.